I found a list of in-depth research articles from This site. Granted, I have not actually read any of them, because the research behind Spaced Repetition (Or as Pimsleur coins it, "Graduated Interval Recall") has never been of particular interest to me.

Pimsleur's "A Memory Schedule" is accessible on the internet here, however it requires you to purchase the right to view the full text of the article.

Besson, M. and Kutas, M. (1993) The many facets of repetition a cued-recall and event-related potential analysis of repeating words in same versus different contexts. Journal of Experimental Psychology: Learning, Memory and Cognition 19, 5: 1115-1133. [11.2]

Bloom, K.C. and Shuell, T.J. (1981) Effects of massed and distributed practice on the learning and retention of second-language vocabulary. Journal of Educational Research 74, 4: 245-248. [11.2]

sugarlevi wrote:The theory behind spaced repitition is based on the way we learn.

Is there actual pedagogical research to support the theory?

Yes there is and cognitive neuroscientific research has provided more information to prove this. Learning theory is a bit more complicated than I put it, there are more factors that are of importance, and there are different ways and theories about how to get something into your long term memory. But rehearsal is still one of the most recognized ways to enter information in your long term memory. Several researchers have proven that mass rehearsal (drilling) is less effective than spaced rehearsal.

And for the neuroscientific research, I do hope everyone has studied their biology, and knows about neurons and action potentials.

Fields, R.D. 2005, (making memories stick) wrote:Some signaling pathways responded quickly and recovered rapidly; thus, they could react to high-frequency patterns of action potentials but could not sustain activation in response to bursts of action potentials separated by long intervals of inactivity. Other pathways were sluggish and could not respond well to rapid bursts of impulses, but once activated, their slowness to inactivate meant that they could sustain signals between bursts of action potentials that were separated by long intervals of inactivity. The genes activated by this pathway would therefore respond to stimuli that are delivered repeatedly, but infrequently, like the repetition necessary for committing new information to memory.